Existing requirements for providing precision approach and landing navigation during flight for both commercial and military aircraft include accuracy, integrity, availability, and continuity of function. Traditionally, location determination incorporates the use of global positioning system (GPS)-based satellite navigation that can provide accuracy down to the centimeter level. The integrity of a navigation system is typically expressed in terms of confidence levels. The higher the confidence level, the more reliable the information provided. Availability and continuity provide assurances that the system will be available not only at the beginning of the operation, but throughout the entire duration of the flight.
Meeting these requirements is especially crucial for autonomous shipboard landings on seaborne aircraft carriers. Proposals of using GPS to generate relative navigation and guidance to meet these challenges can provide the accuracy and integrity required, however, a shipboard approach and landing is more demanding than typical land-based approaches and landings. Aircraft navigation systems used in a shipboard approach and landing must continue to meet the requirements listed above even at sea under severe weather conditions and demanding electromagnetic environments. This is particularly important when landing on an aircraft carrier, where vertical landing errors of more than 0.3 meters is unacceptable and can result in unsafe landing conditions.
Some of the factors to consider during autonomous shipboard landings are a lack of visibility, operating under combat conditions, and a dynamically changing touchdown point. In addition to low rate GPS measurement data other, higher rate, measurements are needed in order to evaluate the relative state between aircraft and aircraft carrier, i.e., the aircraft's position and velocity with respect to the moving runway and touchdown point, as accurately as possible during a precision approach and landing. Existing navigational aids include using an inertial navigation system (INS) to measure the position and altitude of the approaching aircraft in conjunction with GPS. With a combination GPS/INS solution, the short-term measurement data from the INS, which is susceptible to drift errors over time, is corrected by the exact location and time references provided by satellite navigation.
Rapid and high-precision positioning with a Global Navigation Satellite System (GNSS) is feasible only when very precise carrier-phase observations can be used. Raw carrier phase measurements are generally the by-product of all GPS receivers. These phase measurements cannot be used as “range” observations because they are ambiguous.
Carrier phase measurements are ambiguous by an unknown, integer number of cycles. These integer ambiguity parameters need to be resolved before carrier-phase observations can begin to serve as very precise range measurements. For precise navigation, reliable real-time ambiguity resolution is necessary. For short-distance baseline, with current GPS, the reliability of ambiguity resolution with single-epoch data is not high. This makes it impossible to realize real-time precise navigation for safety-related applications. For medium- and long-distance baselines, with current GPS, it generally takes more than twenty minutes to get reliable ambiguity resolution. This low efficiency makes it impossible for global positioning system to be used in many applications where both high precision and high efficiency are needed.